1. Field of the Invention
The present invention pertains to a shock absorbing element for a connecting component, such as a connecting rod, with a flat head, mounted via a ball joint to a pin which is integral with a female yoke. The function of the element is to absorb vibrations between the head and the yoke. The particular use of this invention is for a suspension connecting rod for a module such as the housing enclosing the gears for driving accessory machines on a gas turbine engine applied to the aviation field.
2. Description of the Related Art
A gas turbine engine used for the propulsion of aircraft, a turbojet in particular, includes accessory machines necessary for its operation. Examples are fluid pumps for actuating the control, lubrication or fuel components and also electrical generators. These machines are installed mechanically driven by a gearbox which is itself connected to an engine shaft by a suitable mechanical link. The gearbox is designated by common accord by its abbreviation, AGB for “Accessory Gear Box”, in the field concerned. The gearbox is maintained in suspension on the engine housing slightly downstream of the impeller stage by connecting rod-shaped connecting components.
A connecting rod consists of a rod ending at its two ends by a head of flat shape, with two parallel faces, in the bore of which an externally cylindrical cage is maintained by crimping. The internal surface of the cage acts as a guide to a sleeve with a spherical exterior surface forming a ball-joint. A self-lubricating cloth is glued to the inside of the cage which remains in contact with the spherical surface of the ball-joint to enable relative movements therebetween. This cloth enables the vibration phenomena to be absorbed by an assembly without play and with swivelling torque.
Each connecting rod head 10 is mounted between the lugs 21 and 23 of a female yoke 20 as shown in FIG. 1. In that figure one cannot see the entire connecting rod, only a portion of the head. The head includes two faces parallel to each other 11 and 12, pierced by a bore for the cage housing 13 with its ball-joint 14. The self-lubricating cloth is not shown. The head is maintained between the two lugs of the yoke by a screw 30 which passes through the lugs and the ball-joint. The screw is itself immobilised by a nut 33.
The connecting rod is immobilised axially depending on the direction of the screw 30 by the ball-joint which, as one can see in the figure, is pressing on one side against the internal face of the yoke lug 23 and on the other side against a shoulder 35 of the screw. As a result of this assembly with the ball-joint, the connecting rod head can pivot freely around the screw axis and around any axis perpendicular to the screw axis within the limits of the stops defined by the environment. The movement is limited in particular by the clearance existing on either side between the connecting rod head and the yoke lugs.
During the life-time of the engine, this link is subject to micro-movements due to vibration forces, in particular to stresses orientated parallel to the screw axis, due in particular to the low inclination of the connecting rods compared to the horizontal plane. These stresses in the long term end by wearing and degrading the self-lubricating cloth between the ball-joint and the cage. There follows contact and damage to the metal surfaces which necessitate a repair operation.